Adjustable voltage autotransformer



Nov. 2, 1965 G. o. FREDRICKSQN ADJUSTABLE VOLTAGE AUTOTRANSFORMER 2sheets-sheet 1 INVENTOR.

Filed 00L 1, 1962 United States Patent O 3,215,963 ADJUSTABLE VQLTAGEAUTOTRANSFORMER Gustav O. Fredricksou, Southington, Coun., assignor toThe Superior Electric Company, Bristol, Conn., a corporation ofConnecticut Filed Oct. 1, 1962, Ser. No. 227,277 6 Claims. (Cl. 336-148)The present invention relates to an adjustable voltage autotransformerof the type having an exterior winding formed to provide a commutatingsurface on which a brush is slidably movable in electrical engagementtherewith to provide for adjusting the output voltage by locating thebrush at different positions on the commutating surface.

The heretofore known construction of an autotransformer of the abovetype has generally had a winding toroidally wound as a single layerabout a closed iron core which may be annular. Substantially each turnof the winding has a similar portion bared and preferably flattened sothat the bared portions form a fiat commutating surface on which a brushmay ride to be in electrical engagement with the bared portions to thusconduct current from the winding to an output terminal connected to thebrush. The two end portions of the winding are connected to a source ofalternating current and an output voltage appearing between the brushand an end portion of the winding may be adjusted by moving the brush toa different location on the commutating surface. While such anautotransformer has been found satisfactory there has been a definitelimitation on the maximum power handling capabilities which such anautotransformer may have and yet retain the desirable characteristics.Thus for example, it has been found if the power handling capability isincreased, by increasing the size of the core and also the size and/ ornumber of turns on the winding, that there is a substantial variation inthe voltage between each turn which normally should be susbtantiallyconstant to maintain proper functioning of the brush under loadconditions. Moreover, there is a serious increase of regulation as theoutput current capability increases in addition to a reduction ofeiciency due to a heating of the frame and other metallic parts of theautotransformer that are located adjacent the core. Even without theincrease in power handling capability, the above unwanted effects areproduced if a higher frequency of alternating current is controlled bythe autotransformer and hence the usage of the autotransformer is thuslimited because as the frequency becomes greater the power handlingcapabilities of an autotransformer because of the above effects areaccordingly susbtantially reduced.

Normally while variation in the turn-to-turn voltage between turns wouldnot be a problem, it has been found that in an autotransformer of theabove type it creates a serious effect on the brush by reason of thebrush being designed for a constant turn-to-turn voltage throughout thecommutating surface and that variations therein cause substantialheating by reason of the failure of the brush resistance to fal-l withinthe generally acceptable limitations defined in the Karplus et al.Patent No. 2,009,013.

It is accordingly an object of the present invention to provide anadjustable voltage autotransformer of the above type having an exteriorsingle layer winding formed to provide a commutating surface that is inelectrical engagement with a movable brush in which the abovedeficiencies are substantially minimized or obviated.

Another object of the present invention is to provide in such anautotransformer for the maintenance of the t'urn-to-turn voltage at asubstantially constant value in the turns forming the commutatingsurface when under load.

A further object of the present invention is to obviate the abovedisadvantages enabling the construction of the fects.

3,215,963 Patented Nov. 2, 1965 above-noted type of autotransformershaving larger power handling capabilities and also being able to be usedwith higher frequency alternating current than is heretoforesatisfactorily possible.

In carrying out the present invention, a feature resides in the mannerin which the load current in the winding is distributed over the core sothat the effective or net ampere turns caused by load current in thewinding are substantially minimized. While the brush receives loadcurrent that is the sum of the incremental load current traveling in theexterior winding from one end portion of the winding to the brush andthe incremental load current travelling from the other end portion ofthe brush, these two incremental load currents have not been ablesatisfactorily to sufiiciently minimize or totally cancel each othersampere turns or flux producing ef- According to the present invention,however, this is substantially accomplished by subdividing at least oneof the incremental load currents to the brush into partial load currentsthat travel in different parts of the core and hence spaces the ampereturns produced by one of the incremental load currents throughout amajor portion, if not all of the core. By so subdividing the partialload currents, one of the parial load currents of one incremental loadcurrent may be employed to substantially minimize the eifect of theother incremental load current. Moreover, at particular positions of thebrush and with both incremental load currents being subdivided intopartial load currents, the effects of the load currents may besubstantially completely cancelled.

In one embodiment of the present invention, another or second winding iswound on the core but is wound beneath the exterior winding on which thecommutating surface is formed with the second winding being so inter-`connected with the exterior winding that it carries partial loadcurrents and it carries them in portions of the winding that aredistributed around the core so that a partial lload current in thesecond winding is adjacent to a partial load current in the exteriorwinding. But by the relative direction of winding of the exterior andsecond winding and the direction of travel to the brush of the partialloa-d currents, the two partial load currents are made to produceopposite or cancelling effects on each other and thus substantiallyminimize the net ampere turns in that portion of the core.

The problem of undesirable ampere turns caused by load currents has beenknown in isolation type, i.e., primary and secondary windingtransformers, and results in what has at times been referred to asleakage reactance. However, in maintaining the leakage reactance withdesirable limits the heretofore generally utilized solution requires thestructural interleang of the primary and secondary windings. Inadjustable voltage autotransformers such a solution is not possible notonly in view of the fact that there is no secondary winding but alsobecause of the requirement of having the turns of the exterior windingaccessible so that they may be formed to provide the commutatingsurface.

Other vfeatures and advantages will hereinafter appear.

In the drawing:

FIGURE 1 is an elevation of an adjustable voltage autotransformer inwhich the present invention is incorporated and with a portion thereofshown enlarged.

FIG. 2 is an electrical schematic diagram of one embodirnent of thepresent invention showing the parts and their interconnections and withthe windings being shown as they are physically disposed.

FIG. 3 is a view similar to FIG. 2 but showing a further embodiment ofthe present invention.

Referring to the drawing, an adjustable voltage autotransformer isgenerally indicated by the reference numeral 10 and carries a brushholder 11 on which is mounted abrush 12. The brush holder 11 is mountedfor rotatable movement in a manner well known in the art and thusrotational movement of the brush holder will cause the brush to traversea substantially annular path. The particular embodiment of theadjustable voltage autotransformer in which the present invention isincorporated and herein described is annular 'and includes an annularcore 13 formed from a spirally wound strip of magnetic material.Toroidally wound on the core is an exterior winding 14. The winding 14is of a s-ingle layer as shown and each turn 15 thereof, as illustratedin the enlarged portion of FIG. 1, is formed to provide a bared portion,as indicated by the reference numeral 16. Each bared portion 16 ispreferably made flat, as by grinding, with the bared port-ions 16 ofeach turn positioned to form an arcuate commutating surface 17 that islocated in the path of movement of the brush. The brush is slidablymovable in electrical engagement with the commu-tating surface 17 byengaging the bared portions of one or more turns so that at differentpositions thereof a different value of output voltage is provided. Whilethe core is annular, the exterior winding encircles only about 330thereof and stops (not shown) are provided to prevent the brush holderfrom being so moved as to cause the brush to become disengaged from thecommutating surface.

In carrying out the present invention there is provided beneath theexterior winding 14 another or second winding 18 which in the embodimenthereinafter shown is wound in the same direction and substantiallycoextensive with the winding. In addition, the second winding ispreferably also a single layer winding and made from the same sized wireas the another winding in order that there will be substantially thesame number of turns in each, but as hereinafter will be apparent otherconstruction of the second winding may be employed.

FIG. 2 is an electrical schematic diagram with the windings 14 and 18being pictorially represented as they are physically positioned yon thecore relative to each other. A source of alternating current 19 isconnected to one input terminal 20 and another input terminal 21 of theautotransformer. The brush, indicated by the reference numeral 12, and aload to which the autotransformer may supply an adjusted voltage,depending upon the position of the brush 12 on the winding 14, isindicated by the reference numeral 22 and is interconnected between theterminal 21 and the brush 12. One end position 14a of the exteriorwinding 14 is connected to the terminal 21 while the other end portion14b is connected to the terminal 20. As the autotransformer described inthis embodiment is one whose maximum adjusted output voltage is nolarger than the input voltage, the terminals are connected to the endsof the winding though of course it is within the scope of the presentinvention to connect the input terminals 20 and 21 to the winding 14 atother than its ends to thereby enable the output voltage to be .steppedhigher than the input voltage, as is well known in the art.

The second winding 18 is wound, as heretofore mentioned, as a singlelayer coextensive with but beneath the exterior winding 14 and thus asshown if the winding 14 extends 330, the winding 18 also extends thesame arcuate length. In this embodiment of the invention shown in FIG.Y2, the midpoint 14a` of the winding 14 is connected by a lead 23 to themidpoint 18e of the winding 18 wh-ile one end 18a is connected to theterminal 20 and the other end 18h of this winding 18 is connected to theinput terminal 21.

In the operation of the above described autotransform- `er it will beappreciated that the load current denoted by the arrow Il, to the load22 has an incremental cornponent which ows in the portion of the windingbetween end 14a and the brush 12 in the direction of the arrow and whichincremental load current is denoted by K2IL. In addition, flowing in theportion of the winding 14 between its midpoint 14a` and the brush 12 isanother incremental component of load current K1IL and whose directionis indicated by the arrow adjacent thereto. Moreover, according to thepresent invention the incremental load current KlIL is subdivided intopartial load currents which are caused to flow in different locationsaround the core. Thus there is owing in the winding 18 from the inputlterminal 20 to its midpoint 18C a current denoted K5IL and having itsdirection indicated by the arrow adjacent thereto and also a currentK4IL which flows from the input terminal 21 to its midpoint 18e. Thelead 23 directs the vector sum of these currents to the midpoint 14C ofthe winding 14 where it joins with a current denoted K3IL indicated bythe arrow adjacent thereto and which flows in the winding 14 from theend 14b to the midpoint 14e. The currents K3II, and K5IL both comingfrom the input terminal 20 are Iat the midpoint 14C and 18e joinedvectorially with the current K4IL to become the current KlIL.

There, of Course, is flowing within the windings a magnetizing currentbut according to the present invention while serving to provide tiuxwithin .the core it does not produce the above-noted objectionableeffects and hence for the purposes of the present invention is notmaterial. It is of course understood that the load current issubstantially larger than the magnetizing current.

It will be appreciated that in the particular embodiment herein shownwith the winding 18 being wound in the same direction as the winding 14on the core and each having substantially the same number of turns thatthe value of K5IL current will be substantially equal in amperes to thevalue of K3IL current and thus each will produce the same number ofampere turns. Thus a substantial part of the ampere turns due to theload current are distributed substantially equally over the core insteadof just in one portion thereof. Moreover, with respect to the portion ofthe winding 14 between the end 14a and the brush 12, it will beappreciated that the KSIL current ows oppositely to the K2IL current asdoes IQIL and K3IL current and thus then produce a reduction 1n the netampere turns in the core between the coextensive portion of the windingswhere these currents flow because the ampere turns produced by eachoppose each other and hence reduce the net ampere turns caused by theload current in said coextensive portions of the winding. It will beunderstood that the load current IL is the sum of the incremental KZILand KlIII currents.

While the operation of the autotransformer has been described with thebrush in just one position on the extenor winding 14, it will beappreciated that the operation is the same for the brush in any locationon the wlnding between the end portion 14a and the midpoint 14e. For thebrush located on the other portion of the wlncling 14 between the endportion 14b and the midpoint 14e, the operation will be identical onlythe incremental load current from the end 14a to the brush will besubdivided to have a portion of it flow through the part of the winding18h to 18e1 while the incremental load current from the end 14b flows tothe brush and is subdivided to have a portion flow in the part 18a to18C of the winding 18.

While the above structure has been found to reduce the above-noteddeficiencies, it has been found that a more equal distribution of theampere turns caused by the load current may be made by the embodiment ofthe invention shown in FIG. 3. Here a more equal distribution withgreater minimizing of the deficiencies is achieved and also there is anincrease in the power handling capability in the adjustable voltageautotransformer. In this embodiment rather than have both the exteriorwinding and the underlying winding divided into just two portions, thisembodiment of the invention divides the windings into six portions andin addition has, as heretofore mentioned, a step-up portion so that theoutput voltage of the transformer may be adjusted to a value higher thanthe input Voltage. It will of course be appreciated that according tothe present invention division of the six is merely arbitrary and anynumber may be selected up to and including a division equal to thenumber of turns in the exterior winding if so desired depending upon thepower handling requirements, frequency of the alternating current sourceand the criticalness of maintaining a constant turn-to-turn voltage. f

In the embodiment of the invention shown in FIG. 3 the commutating orexterior winding is indicated by the reference numeral 30 and has oneend 30a connected to an input terminal 31. There are provided taps 301),30C, 30d, 30e, 30]c and 30g with the tap 30g being connected to anotherinput terminal 32 and an end 30h. Similarly a secondary winding 33 iswound coextensive with the winding 30 on the core but beneath it has anend 33a connected to the input terminal 31 and taps formed thereinindicated by reference characters 33b, 33C, 33d, 33e, 33f and 33g, thelatter being connected to the other input terminal 32, and an end 33h.Preferably the taps on each winding subdivide the windings between theirconnections to the input terminals into portions having an equal numberof turns. Thus, for example, if the windings 30 and33 both have 192turns between the ends 30a and 33a and the taps 30g and 33g respectivelythen the taps are spaced to include 32 turns. The overvoltage portionbetween each winding between the tap 30g and 33g and the end 30h and 33hmay also have this number of turns if so desired but need notnecessarily.

With the above structure it will be appreciated that the same result asachieved with the embodiment of the invention in FIG. 2 is attained butthere is a more even distribution of the ampere turns due to the loadcurrent about the winding. With a brush 34 located in the position shownin FIG. 3 between the taps 30e and 30f, there are incremental loadcurrents flowing thereto as indicated by the arrows reference charactersKlIL and K2IL and the direction of the current is shown by the arrowsadjacent their respective reference characters. Moreover the incrementalload current KlIL is the sum of the partial load current K3IL flowing inthe winding 30 from the tap 30g to the tap 30]c and the partial loadcurrent K5IL owing between the taps 33g and 33]c which through theconnection to the tap 30jc effects the combination of these two currentsto the incremental load current KlIL. The other increment of loadcurrent K2IL is also subdivided into a partial load current K4IL fiowingin the winding 30 from the end 30a to the tap 30e and a partial loadcurrent KGIL which flows to the tap 30e from the tap 33e and is derivedfrom the end 33a. It will be appreciated that KSIL substantially equalsKlll and K4IL substantially equals KGIL and in addition KIIL-i-KZILequals IL in View of both windings 30 and 33 in this embodiment havingthe same number of turns and being interconnected inthe manner shown.

For other positions of the brush, the incremental load currents willalso be subdivided to the taps of the exterior winding nearest thebrush. Whenever there is a partial load current flowing in a part of thesecond winding 33 of one incremental load current, it ows in a directionthat produces load current ampere turns that are opposite and cancel thepartial load current -fiowing in the exterior winding of the otherincremental load current. The parts of the two windings in which theopposing currents flow are moreover at least adjacently disposed to eachother on the core while, in fact, generally the part of the exteriorwinding is specifically overlying the part of the second winding.

It will accordingly be appreciated that there has been disclosed anadjustable voltage autotransformer which while retaining the advantagesof such an autotransformer obviates many deficiencies which haveseverely limited its use. Thus the present invention enables a higherfrequency of alternating current to be controlled and/or have a largerpower handling capability. Moreover, by the use of such a constructionof an adjustable voltage autotransformer, disclosed herein, theturn-to-turn voltage is substantially made constant throughout thecommutating turns of the exterior winding and better regulation and anincrease in efciency are obtained.

Variations and modifications may be made within the scope of the claimsand portions of the improvements may be used without others.

I claim:

1. An adjustable voltage autotransformer comprising a closedmagnetizable core, an exterior winding wound on said core to have turnsencircle the core and a pair of end portions, said exterior windingbeing a single layer winding and having a commutating surface formed onthe turns, a brush mounted for movement on said commutating surface tobe in electrical engagement therewith, a pair of input terminalsconnected to the end portions of the exterior winding, a pair of outputterminals with one being electrically connected to the brush and theother to an end portion of the winding, a second winding wound on saidcore, and means interconnecting the second winding to the exteriorWinding and the input terminals for subdividing an incremental loadcurrent that fiows to the brush from one input terminal into a partialload current which flows through one part of the second winding andanother partial load current which flows through one part of theexterior Winding at substantially all positions of the brush, said partof the second winding being connected in parallel with said one part ofthe exterior winding.

2. The invention as defined in claim 1 in which the one part of thesecond winding and the part of the exterior winding are normally locatedon different places of the core.

3. The invention as defined in claim 1 in which the one part of thesecond winding and the one part of the rgerior winding are wound on thecore in a direction with respect to the direction of current flowtherein which causes the partial load current flowing in the one part ofthe second Winding to flow in a direction which produces load currentampere turn fiux that is the same direction as the load current ampereturn flux produced by the partial load current flowing in the one partof the first winding.

4.. The invention as defined in claim 1 in which another incrementalload current flows to the brush from the other input terminal throughanother part of the exterior winding, and wherein the one part of thesecond winding and the another part of the exterior winding areadjacently disposed on the core and wound in a direction with respect tothe current flowing therein that causes the partial load current flowingin the one part of the second winding to produce ampere turns which areopposite to the incremental load current flowing in the another part ofthe exterior winding.

5. The invention as defined in claim 1 in which the interconnectingmeans includes at least one tap on the exterior winding subdividing theexterior winding into the one and another part with each havingsubstantially the same number of turns, a tap on the Isecond windingsubdividing the winding into the one and another part with each havingsubstantially the same number of turns and the interconnecting meansincludes a connection between the two taps.

6. The invention as defined in claim 1 in which the interconnectingmeans includes the exterior winding having a plurality of tapssubdividing the exterior winding into a plurality of parts including theone part with each part having substantially the same number of turns,the

7 8 second winding having a plurality of taps subdividing the ReferencesCited by the Examiner second Winding into a plurality of parts includingthe one UNITED STATES PATENTS part with each part having substantiallythe same num- 3 O87 132 4/63 Snowdon et al 336 148 ber of turns, therebeing the same number of taps on each winding and the interconnectingmeans further includes a 5 FOREIGN PATENTS plurality of connections witheach connection being only 857,141 12/60 Great Brltam.

from a tap of the exterior Winding to atap of the second LARAMIE E.ASKIN, Primary Examiner.

winding. JOHN F. BURNS, Examiner.

1. AN ADJUSTABLE VOLTAGE AUTOTRANSFORMER COMPRISING A CLOSEDMAGNETIZABLE CORE, AN EXTERIOR WINDING WOUND ON SAID CORE TO HAVE TURNSENCIRCLE THE CORE AND A PAIR OF END PORTIONS, SAID EXTERIOR WINDINGBEING A SINGLE LAYER WINDING AND HAVING A COMMUNATING SURFACE FORMED ONTHE TURNS, A BRUSH MOUNTED FOR MOVEMENT ON SAID COMMUTATING SURFACE TOBE IN ELECTRICAL ENGAGEMENT THEREWITH, A PAIR OF INPUT TERMINALSCONNECTED TO THE END PORTIONS OF THE EXTERIOR WINDING, A PAIR OF OUTPUTTERMINALS WITH ONE BEING ELECTRICALLY CONNECTED TO THE BRUSH AND THEOTHER TO AN END PORTION OF THE WINDING, A SECOND WINDING WOUND ON SAIDCORE, AND MEANS INTERCONNECTING THE SECOND WINDING TO THE EXTERIORWINDING AND THE INPUT TERMINALS FOR SUBDIVIDING AN INCREMENTAL LOADCURRENT THAT FLOWS TO THE BRUSH FROM ONE INPUT TERMINAL INTO A PARTIALLOAD CURRENT WHICH FLOWS THROUGH ONE PART OF THE SECOND WINDING ANDANOTHER PARTIAL LOAD CURRENT WHICH FLOWS THROUGH ONE PART OF THEECTERIOR WINDING AT SUBSTANTAILLY ALL POSITIONS OF THE BRUSH, SAID PARTOF THE SECOND WINDING BEING CONNECTED IN PARALLEL WITH SAID ONE PART OFTHE EXTERIOR WINDING.